1. Field of the Invention:
The invention relates in general to the field of refrigeration having to do with the cooling of air; and in particular to the design of air cooling heat exchangers or coils on which frost deposits during the cooling function; and to the electrical defrosting mechanism which is employed to periodically warm the coils to thaw the frost, leaving the coil in frost-free condition for continued refrigeration at optimum efficiency.
2. DESCRIPTION OF THE PRIOR ART
Refrigeration air cooling units and evaporators having electric defrost heaters have been known for many years. Since frost normally deposits uniformly over the body of the coil, designers have distributed the defrosting heat uniformly over the body of the coil. Experiences have shown that when the coil is warmed by the defrost heaters, the warmed air within the fins tends to rise through the fin pack of the coil by gravity and flow into the cold room. This gravity circulation of air warmed by the defrost heaters has two harmful effects: first, the moisture carried by the warmed air deposits on the internal or external portions of the cooling unit and on the ceiling of the freezer causing frost deposition; second, as air flows out of the coil by convection, fresh, cold air from the freezer enters the coil at the bottom of the fin pack, cooling it and delaying or even defeating the defrosting at that lowest portion. Some designers of refrigeration evaporators have gone so far as to provide movable doors to isolate the evaporator from the freezer during the course of defrost to inhibit this effect. Automatic, movable doors, however, have not always proved to be completely reliable mechanically and have sharply increased the cost of the assembly.